PSI - Issue 75

Gary B. Marquis et al. / Procedia Structural Integrity 75 (2025) 530–537 Marquis, Barsoum & Leitner / Structural Integrity Procedia (2025)

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• Introduction of numerical fatigue strength modification factors. The 2024 guidelines introduce quantitative fatigue strength modification factors to reflect the influence of yield strength and load stress ratio (R-ratio). These factors allow for a continuous improvement estimation, as an alternative to the stepwise increase in fatigue class with yield strength from the 2016 edition.

• Updated thickness correction factors aligned with the 2024 IIW Fatigue Design Guidelines [9]

• Refined S–N curve descriptions. The updated guideline includes detailed recommendations on selecting appropriate S–N curves for HFMI-treated joints based on steel strength, loading conditions, and stress assessment method (nominal, hot-spot, notch stress). These are now presented both as rules and as tables and figures for easy reference, improving usability and clarity. • Inclusion of guidelines for retrofitting pre-fatigued structures. For the first time, the guideline includes a dedicated chapter on the use of HFMI in rehabilitating existing structures such bridges and other large structures, where fatigue damage has already occurred. Treatment is allowed for crack depths up to 1.5 mm, provided certain quality control and assessment procedures are followed [9]. • Standardized verification methodology for HFMI devices. The updated document introduces a methodology for validating the effectiveness of different HFMI devices based on experimental testing and documentation. This approach acknowledges the diversity of equipment (e.g., ultrasonic peening, pneumatic impact tools) and aligns with IIW's non-commercial status. • Updated recommendations for corrosive environments. The guideline highglights that HFMI-treated joints require appropriate corrosion protection to maintain fatigue performance in atmospheric or marine environments. The document provides best practices and highlights degradation risks in unprotected conditions. 3. Treatment Procedures and Quality Control High-Frequency Mechanical Impact (HFMI) treatment enhances fatigue strength by improving weld toe geometry and inducing compressive residual stresses. Its effectiveness depends on operator skill, proper application, and reliable quality control. Operators must be trained on the specific HFMI tool and relevant weld details. Training is device-specific and often includes practice on representative welds. Complex geometries may require advanced training and formal qualification (e.g., ISO 14732) may be required for critical applications. The weld toe must be clean, de-slagged, and accessible. HFMI is only effective when applied directly at the weld toe figure 2 and improper treatment, such as on steep angles or large reinforcements, may cause embedded defects. Light grinding may be used to reduce these risks but must not obscure the weld toe. Earlier ISO 5817 weld profile demands have been relaxed. Welds meeting IIW fatigue class requirements are sufficient, even with profile variation. Qualitative Control: for qualitative control a correct groove is smooth, shiny, and continuous, as shown in figure 2, where crack like features or visible fusion lines indicate poor treatment and require rework. For quantitative control, typical groove dimensions are 0.05–0.6 mm deep and 2–6 mm wide, figure 2.

Figure 2. Example of a smooth, well-formed HFMI groove with closely spaced impacts, showing relevant indentation depth parameters.